Nail on head:
...pilots complain is because the reality does not come close to the preconceptions that they had of the job.
And sadly, the past few years have got worse, mainly due to a crazed bonus culture that seems to have swept through management teams around the UK...and pilots are high cost...so I would also add to the above and say
not so much the preconceptions of doing the actual day job of aviating (quite a few years in and I still get a kick out of it!) but a huge disappointment in the industry as a whole...
I think this will be the hardest thing for a Wannabe to get his/her head around if they are choosing a second career and you have experience of another industry.